Convert Square Millimeter to Square Inch
Convert square millimeters to square inches instantly. 1 square millimeter = 0.0015500031 square inch — use the live calculator, the exact formula, a conversion table and worked examples. Also check the Square Inch to Square Millimeter converter for the reverse conversion.
Units explained
Square Millimeter
A square millimeter is a metric unit of area equal to 10⁻⁶ m² (1 mm × 1 mm). It is widely used in engineering, electronics, and small-scale technical work.
Derived by squaring the millimeter, defined as 1/1000 of a meter. The milli- prefix comes from the Latin 'mille' (thousand).
Square millimeters express PCB trace cross-sections, the area of small mechanical features in CAD drawings, biological sample sizes, and material thicknesses.
Millimeter has been part of the metric system since 1795.
Square Inch
A square inch is an imperial unit of area equal to exactly 0.00064516 m², the area of a square with sides of one inch. It is widely used in the United States for small-area measurements.
Derived by squaring the inch. The inch was standardized to exactly 25.4 mm via the 1959 International Yard and Pound Agreement.
Square inches are used extensively in US engineering, manufacturing, real estate (for floor plans), and everyday small-area measurements. Engine displacement is often expressed in cubic inches but cross-sections in square inches.
Inch dates to medieval English usage; modern exact value standardized in 1959.
Square Millimeter to Square Inch conversion formula
The relationship between square millimeters and square inches:
To convert square millimeters to square inches, multiply the value in square millimeters by 0.0015500031. To reverse, multiply square inches by 645.16.
How to use this converter
Type a value into the calculator. The result in square inches updates as you type. Tap a quick value, copy the result with one click, or use the swap arrow to jump straight to the Square Inch to Square Millimeter converter for the reverse direction.
Step-by-step: convert square millimeters to square inches
- Write down the value in square millimeters (mm²).
- Multiply that value by the factor 0.0015500031.
- The product is the equivalent value in square inches (in²).
- To reverse, multiply the square inch value by 645.16.
Worked examples
Example 1 — Convert 1 mm² to in²:
1 × 0.0015500031 = 0.0015500031 in²
Example 2 — Convert 100 mm² to in²:
100 × 0.0015500031 = 0.15500031 in²
Real-world example — Plastic-film thickness alternates
A 150-square millimeter plastic film converts cleanly to square inches — useful for packaging buyers reconciling supplier datasheets across metric and US measurement systems.
150 mm² × 0.0015500031 = 0.232500465 in²
Real-world example — Hair-width scale measurements
A 70-square millimeter measurement (about the diameter of a human hair) is the kind of value materials engineers regularly express in adjacent micro-scale units like square inches for direct comparison across supplier datasheets.
70 mm² × 0.0015500031 = 0.108500217 in²
Real-world example — Paper and film thicknesses
At the thickness of office paper (roughly 3 square millimeters), converting between sub-millimeter units is routine for packaging and printing buyers comparing quotes from metric and US suppliers.
3 mm² × 0.0015500031 = 0.0046500093 in²
Square Millimeter to Square Inch conversion table
Standard reference values for converting square millimeters to square inches:
| Square Millimeter [mm²] | Square Inch [in²] |
|---|---|
| 0.01 | 1.550003e-5 |
| 0.1 | 0.0001550003 |
| 1 | 0.0015500031 |
| 2 | 0.0031000062 |
| 3 | 0.0046500093 |
| 4 | 0.0062000124 |
| 5 | 0.0077500155 |
| 10 | 0.015500031 |
| 20 | 0.031000062 |
| 30 | 0.046500093 |
| 40 | 0.062000124 |
| 50 | 0.077500155 |
| 100 | 0.15500031 |
| 500 | 0.77500155 |
| 1000 | 1.5500031 |
Frequently asked questions
How many square inches is 1 square millimeter?
How do I convert square millimeters to square inches?
How do I convert square inches back to square millimeters?
How many square inches is 100 square millimeters?
Popular area unit conversions
Convert Square Millimeter to other area units
Show all Square Millimeter conversions
Metric / SI (15 units)
Imperial / US Customary (15 units)
US Survey (5 units)
Indian Subcontinent (16 units)
Other Regional (10 units)
Scientific / Physics (5 units)
Sources & references
Conversion factor (1 mm² = 0.0015500031 in²) verified against the following authoritative sources:
- BIPM — The International System of Units (SI Brochure 9th ed.)
Official BIPM publication defining the seven SI base units (including the meter) and the rules for their use. The global authority on units of measurement.
- NIST — Guide to the SI
US National Institute of Standards and Technology reference covering the SI base and derived units with definitions and usage rules for US technical practice.
- NIST Special Publication 811 — Guide for the Use of the International System of Units
Detailed NIST guide covering exact conversion factors between SI and US customary units along with formatting and rounding conventions.
- NIST — Refinement of values for the yard and pound (Federal Register 1959)
The treaty (signed by US
- International Astronomical Union — System of Astronomical Constants
The IAU defines astronomical units including the AU (149597870700 m exactly) light-year and parsec used in astronomy and astrophysics.